1. Technical Field
The present invention relates to surgical instruments useful in a sub-specialty of the field of plastic surgery referred to as craniofacial surgery.
2. General Background
Craniofacial surgery deals with surgical reconstruction of the skeleton of the face and cranium and the reconstruction of other like bony structures. There are certain advantages in the use of a cranial bone graft for facial reconstruction. The donar area is substantially painless as compared to sites such as the rib or the hip. The quality of bone graft material is superior for use in reconstruction, as it is more readily incorporated into the recipient bed and there is less resorption during healing. The harvesting of bone graft material from this area usually produces less surgical dissection overall, as the same exposure can be used for both the reconstruction and for the harvesting of the bone graft. This latter advantage also results in a saving of operating time.
Previous techniques have been described for the harvesting of bone grafts from the cranium for use in craniofacial surgical reconstruction. These techniques have included inter alia, using a neurosurgical drilling device called a "cranial perforator" to harvest "bone dust" by drilling through the outer two layers of the skull at multiple sites. Some techniques have used a standard osteotome (basically, a chisel) having either a straight or curved blade with a free hand technique to tangentially chip off sections of the outer layer of the cranium. Neurosurgical devices have been used to cut out a full thickness piece of skull. This entire section of the cranium is then taken to a separate working area in the operating room where it is split between the inner and outer tables using sharp chisels. Either the inner or the outer layer can then be used for the surgical reconstruction. The remaining layer is returned to the donar site to fill in the hole left in the cranium.
Problems exist with prior art devices and methods of harvesting bone graft material from the cranium. Cranial perforators only harvest a bone powder material. This type of powder material is difficult to use because it is not rigid. It can only be used as a "packing" substance and cannot be used to make solid forms. While the method of using a free chisel to tangentially chip off the outer layer provides solid pieces of bone suitable for a wider range of reconstructive procedures, this method suffers from two problems. Firstly, the surgeon has no way of visualizing the depth of the cut he is making, thus the procedure is "blind." In addition, there is a dangerous possibility of the blade penetrating through the cranium and into the brain tissue below. Due to such a risk, this "chisel" technique has not found wide acceptance. The third method of splitting a full thickness plate of cranium harvested by neurosurgical techniques, has the advantage of providing large pieces of solid bone graft material for the surgical reconstruction. It suffers in that an extensive neurosurgical procedure is required to harvest the original full thickness plate of cranium. Besides being time-consuming it is also not free of risk. Further, this procedure usually requires a second surgical team, adding complexity and expense.
General Discussion of the Present Invention
The present invention solves these prior art problems and shortcomings in a straightforward, simple yet effective manner by providing a guided osteotome. The guided osteotome is an instrument that harvests bone graft material in rigid pieces of pre-determined, generally uniform thickness. The device includes a blade that is preferably longitudinally curved away from the curve of the surface of the cranium to impede the blade's tendency to "plunge" as it advances. The guided osteotome of the present invention has laterally extending guides having bearing surfaces which "track" the surface of the cranium during harvesting, thus physically limiting the depth of cut and thereby preventing the cutting element from plunging deeper than a predetermined known depth which also desirably defines the thickness of a layer of bone graft material harvested. The guides also can project forward of the cutting edge allowing the surgeon to visually monitor linear progression of the cutting edge along the skull during cutting. The device can be used without the need of supportive neurosurgery techniques that accompany exposure of the underlying brain.
The present invention thus provides a guided osteotome that is both safer and easier to use for harvesting cranial bone grafts. The apparatus features a shaft with a handle supporting a longitudinal curved blade having a longitudinal curve opposite the curve of the outer surface of the cranium. The blade thus lessens the tendency for the blade cutting edge to plunge to a deeper depth as it advances.
The guided osteotome of the present invention is used in the harvesting of bone graft for craniofacial plastic surgery. The apparatus includes an elongated instrument body having a blade which includes a transverse cutting edge for cutting through a layer of selected cranial bone tissue suitable for use as a graft. The blade includes a flattened portion adjacent the transverse cutting edge. One or more guides are positioned on the blade, preferably on opposite edge portions thereof for defining the depth of cut. The guide includes one or more bearing surfaces which track upon the surface of a patient's skull during the bone graft harvesting operation. In the preferred embodiment, two spaced apart guides are disposed generally on opposite edge portions of the blade. In the preferred embodiment, the guides include a pair of spaced apart guides, each of which has a first bearing surface generally parallel to the cutting edge which is that surface that rides upon the surface of the patient's skull during the bone graft harvesting. A second surface extends angularly (preferably at right angles) between the first bearing surface and the transverse cutting edge, thereby forming a connection between the first bearing surface and the flattened portion of the blade. In the preferred embodiment, the guides are in the form of one or more projections which extend forwardly of the transverse cutting edge. Each of the forwardly extending projections is rounded in cross-section in the preferred embodiment. The transverse cutting edge can be curved, corresponding to the curvature of the patient's skull. This latter embodiment is especially useful in making wider cuts to harvest wider sections of graft material.
It has been found that for the human skull an exemplary radius of seven (7.0) centimeters over 22 degrees for the cutting edge is useful. The blade can then be tapered straight over the next 22 degrees. Because the surface of the human skull is curved in two directions, it has also been found that if the blade is wider than one (1.0) centimeter, precautions must also be taken to stop the center portion of the blade from penetrating the skull "inner table." It is necessary in the wider blades to give the transverse aspect of the cutting edge a curverature to match that of the cranium to avoid this problem. The most efficacious transverse curvature is radius 7.0 centimeters. This can either be accomplished by curving the bottom of the blade itself or by sharpening the end as a curve so that the center of the blade is withdrawn up the curve of the shaft and is functionally higher than the sides during the cutting process.
To give control to the depth of the cut and in particular to prevent application through the cranium, the sides of the blades are also provided with the aforementioned guides. These guides act like small skids that ride along the surface of the outer table of the cranium while the blade cuts below. These guides are manufactured at standard preset depths, for example, one (1.0), two (2.0) or three (3.0) millimeters, giving the surgeon several options as to the thickness of the bone graft material he can harvest.
Pre-operatively the surgeon will check the thickness of the skull by X-ray before deciding upon a thickness for the graft material. Not only do the guides act to physically impede plunging of the blade into the skull, but they are a fixed point of reference in relation to the cutting edge. The surgeon can thus visually monitor these two points as the cut progresses longitudinally so that the cutting action is not done blindly. The guides include projections which can be designed to project forward beyond the front of the cutting edge a predicted distance so that, for example, the angle between the line of the longitudinal end tangent of the blade and the line between the tip of the guides and the tip of the blade is forty-five degrees (45%).
The technique of use of the present invention follows: Following an X-ray to estimate the thickness of the skull outer table, two parallel cuts are made along the proposed path of the osteotome through the outer table of the cranium. This is done using any type of cranial saw. The cuts are parallel and spaced apart a distance equal to the width of the osteotome to be used. The depth of the saw cuts is through the outer table only. The surgeon can visually judge this depth because the diploe is a different color then the outer table bone. Once this has been done, a cross cut is made at right angles, or ninety degrees (90%) to the longitudinal cuts and at one end thereof (FIG. 5). The right angle cut is made to the same depth and joins the two parallel cuts. Once the outer table has been prepared, the cutting edge of the osteotome is placed in the cross cut and a mallet or hammer is used to drive the osteotome along the path of the two parallel side cuts. The two parallel cuts prevent the osteotome from skidding laterally of the projected path. The two guides slide along the skull surface preventing the blade from plunging into the skull beyond the predetermined depth. In addition, the surgeon visually monitors the progress of the transverse blade cutting edge. Once the blade has been passed and the bone graft taken, bone wax is applied to the raw surface of bone (i.e., the upper surface of the inner table) to control bleeding. No further reconstruction of the donar defect is necessary.
The instrument body could be a frame supporting a power driven cutting element having a powered blade such as a reciprocating or band saw blade, for example. The guides could be thus supported by the frame so that the cutter would likewise harvest bone graft material of a uniform defined depth. A reciprocating blade would oscillate while travelling along the path of two parallel cuts made in the outer table of the skull and spaced apart a distance equal to or greater than the excusion of oscillation.